1. Technical Field
The present invention relates to a Micro Electro Mechanical System (MEMS) device.
2. Related Art
MEMS devices manufactured by using MEMS technology have recently been drawing great attention. Such MEMS devices include a minute MEMS formed on a semiconductor substrate so as to be utilizable for sensors or resonators. The MEMS is provided with a fixed electrode and a movable electrode. By bending the movable electrode, an electrostatic capacitance generated at the fixed electrode is detected to thereby provide MEMS characteristics.
In general, it has been known that parasitic capacitance included in some circuit wirings such as ICs will adversely affect the electrical characteristic of ICs and the like. Parasitic capacitance also occurs in MEMS devices. An adverse effect on the electrical characteristic caused by the parasitic capacitance is worsened as the space between electrodes in the MEMS becomes narrower and the applied frequency becomes higher.
Parasitic capacitance is easily formed between the semiconductor substrate and the MEMS when the MEMS is produced through a surface MEMS process in which the MEMS is directly formed on an extremely thin oxide film or nitride film on a semiconductor substrate This is true even when the MEMS occupies a small area.
In particular, an electrostatic type MEMS device for detecting volume displacement generated by mechanical displacement of a movable electrode has an extremely weak output signal. In addition, since an absolute value of the volume displacement is not sufficiently large with respect to parasitic capacitance, the signal is easily affected by the parasitic capacitance.
Further, when the parasitic capacitance is large and the resistance of a surface of the substrate is small, or when a capacitance between the substrate and an electrode is large, the signal is easily leaked from pathways other than the original pathway through carriers excited on the surface of the substrate.
For example, FIG. 16 illustrates a known MEMS device that includes a MEMS formed on an oxide film 111 and a nitride film 112 on a semiconductor substrate 110. The MEMS device is provided with fixed electrodes and a movable electrode. The fixed electrodes include an input electrode 113, an output electrode 114 and a driving electrode 115. The movable electrode includes a movable portion 116 coupled to the input electrode 113.
In a MEMS device having the above structure, a high-frequency signal may leak from the input electrode 113 to the output electrode 114 through the surface of the semiconductor substrate 110.
To solve this problem, JP-A-2006-174174 (page 5, lines 7 to 11) discloses a leakage amount reduction technique for a high-frequency signal to a substrate by collectively and commonly coupling lower electrodes of a resonator element (MEMS) so as to reduce an area occupied by the wiring of the high-frequency signal.
However, although reducing the area occupied by the MEMS as described above is an effective method for decreasing parasitic capacitance, reducing the occupied area is not always easily accomplished due to restrictions of designs and/or production. Therefore, when the area occupied by the MEMS is not capable of being successfully reduced, the parasitic capacitance causes an adverse effect to the characteristics of the MEMS device.